Conventional calendar display devices for timepieces, and in particular wearable wristwatches, usually use an annular date-disc including 31 regularly distributed annular sectors, each of the sectors including a symbol or indication corresponding to one of the possible dates of the month. The date-disc is indexed by one position every day, so as to reveal through an aperture the date value corresponding to each day, which is formed of one or more numerals. The angular sector occupied by each indexing position is thus relatively limited (360/31 i.e. slightly less than 12 degrees), which greatly restricts the maximum size of the numbers that can be displayed therein.
In order to increase the size of the symbols displayed in the aperture for reading the date, there therefore exist systems using two distinct discs, one for the display of the tens numeral and the other for the display of the units numeral of the date. This type of calendar display combining two numerals borne by two distinct discs is referred to as a “grand date” display. These display devices are coupled to a control device for respectively indexing each of the discs in order to display every day the exact combination of units and tens.
There is known, for example, a “grand date” display mechanism including a units ring comprising the units sequence 0-9, and a tens disc including a sequence of four numerals 0-3 distributed over sectors of approximately 90° each, like that described in EP Patent No 2490083. A date programme wheel with 31 sectors, driven at a rate of one step per day, meshes on two separate planes respectively with a tens disc and a units ring, to respectively drive the units disc every day, except on the change from the 31st day of the month to the first day of the following month, by means of 30 teeth followed by a toothless sector—the 31st of the date programme wheel—and the tens disc is only driven four times per month for the change to the next ten and the change of month (9->10, 19-20, 29-30 and 31->01) by means of long teeth respectively arranged on the 9th, 29th and 31st sectors of the 31 gear sectors of the programme wheel, and which drives a cross integral in rotation with the tens wheel. This date programming mechanism thus uses two distinct kinematic chains from the date programme wheel, driven at a rate of one step per day by a 24 hour wheel, without any mutual interaction between the tens display disc and the units display disc. Indeed, each of the 31 gear sectors of the date programme wheel meshes selectively plane-by-plane with each of the discs concerned to display each date value throughout the month.
One drawback of this type of calendar mechanism is that it requires a very large number of parts, which are separate for the control and display mechanism, and therefore occupy too much space on the plate. Further, the cross associated with the tens disc has a very small number of teeth, which is detrimental to gearing reliability due to the large angular steps required during each indexing operation.
Other types of “grand date” display mechanisms are also known, for example from EP Patent No 1609028, using a combined solution with a modified tens disc that can sometimes contain two numerals for certain dates. This solution discloses a device for coupling the disc bearing the units to that bearing the tens, the units disc bearing lugs for driving the tens disc on certain dates. According to this display solution, the fact that the modified tens disc bears a larger number of symbols certainly facilitates the gearing mechanism, but because the sequence of symbols relating to the units is arranged on a disc integral in rotation with a programme wheel having 31 sectors, the maximum size of the characters that can be displayed consequently still remains very limited.